1. Field of the Invention
The present invention relates to a lithographic printing plate precursor and to a method of lithographic printing that uses this lithographic printing plate precursor. More particularly, the present invention relates to negative-working lithographic printing plate precursor that enables direct platemaking in which platemaking is carried out directly based on a digital signal from, e.g., a computer, by scanning with, for example, a laser having a wavelength from 300 to 1200 nm. The present invention further relates to a method of preparing a lithographic printing plate in which the aforementioned lithographic printing plate precursor is developed directly on the press without going through a development processing step, and a method for lithographic printing in which printing is carried out on the press accordingly.
2. Description of the Related Art
A lithographic printing plate typically comprises an oleophilic image area that is ink receptive during the printing process and a hydrophilic nonimage area that is fountain solution receptive during the printing process. Lithographic printing is a method that utilizes the fact that water and oleophilic ink repel each other: differences in the ink attachment behavior are produced on the surface of the lithographic printing plate by using the oleophilic image areas on the lithographic printing plate as ink receptive areas and using the hydrophilic nonimage areas on the lithographic printing plate as fountain solution receptive areas (areas not receptive to ink). After ink uptake has been brought about only in the image areas, the ink is transferred to the receiving medium, e.g., paper.
A lithographic printing plate precursor (PS plate) comprising an oleophilic photosensitive resin layer (image recording layer) disposed on a hydrophilic support has heretofore been widely used to produce the aforementioned lithographic printing plate. Platemaking is typically carried out by a method in which the lithographic printing plate precursor is exposed to light through an original image, for example, a lith film, after which the areas forming the image areas of the image recording layer remain while the unwanted image recording layer outside these areas is dissolved and removed by an alkaline developing solution or an organic solvent to expose the hydrophilic surface of the support, thus yielding the lithographic printing plate.
The conventional platemaking process for lithographic printing plate precursors has required a step in which, after photoexposure, the unwanted image recording layer is dissolved and removed by, for example, a developing solution adapted to the image recording layer; however, a concern with these separately conducted wet processes has been to render them unnecessary or to simplify them. In particular, attention to the global environment has in recent years caused the disposal of the waste solutions discharged in association with these wet processes to become a major issue for the industrial sector as a whole, and as a consequence there has been an even stronger desire to address the aforementioned concern.
In this context, the method known as on-press development has been introduced as a convenient platemaking method. In on-press development, an image recording layer is used that enables the removal of unwanted areas of the image recording layer to be carried out during an ordinary printing process: after photoexposure, the lithographic printing plate is obtained by removal of the unwanted areas of the image recording layer on the press.
The following are examples of specific methods of on-press development: use of a lithographic printing plate precursor that has an image recording layer that can be dissolved or dispersed in the fountain solution, in the ink solvent, or in an emulsion of the fountain solution and ink; mechanical removal of the image recording layer by contact with rollers or the blanket cylinder on the press; mechanical removal of the image recording layer by carrying out contact with rollers or the blanket cylinder after the cohesive strength within the image recording layer or the adhesive force between the image recording layer and support has been weakened by penetration by, for example, the fountain solution or the ink solvent.
Unless stated otherwise, in the present invention, the “development processing step” refers to a step in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer that have not been exposed to the laser, wherein this removal is effected by contact with a fluid (typically an alkaline developing solution) using an apparatus (typically an automatic developing apparatus) outside of the press, and “on-press development” denotes a step and a method in which the hydrophilic surface of the support is exposed by the removal of those areas of the image recording layer that have not been exposed to the laser, wherein this removal is effected by contact with a fluid (typically the printing ink and/or fountain solution) using the press.
At the same time, digital technology, in which the image data is electronically processed, stored, and output using a computer, has become widespread during the last few years, and various new image output methods have entered into practice in association with this digital technology. Accompanying this, interest has been growing in computer-to-plate (CTP) technology, in which the digitized image data is carried by a highly convergent beam of radiation, for example, laser light, and the lithographic printing plate precursor is subjected to a scanning photoexposure with this light in order to directly produce the lithographic printing plate without going through lith film. As a consequence, the appearance of lithographic printing plate precursors adapted to this technology has become a technical problem of the utmost importance.
Thus, as described in the preceding, based on global environmental concerns and the need to adapt and conform to digital technology, there has recently been an even stronger desire than before for a simplification of platemaking technology, for its conversion to a dry technology, and for its conversion into a processless technology.
Within the sphere of lithographic printing plate precursors, a scanning-photoexposable lithographic printing plate precursor comprising a hydrophilic support bearing an oleophilic photosensitive resin layer that contains a photosensitive compound that can produce an active species (e.g., a radical or a Bronsted acid) upon laser photoexposure has been introduced and has already appeared on the market. An active species can be produced by scanning this lithographic printing plate precursor with a laser based on digital data, and this action causes a physical or chemical change in the photosensitive layer, which induces insolubilization. A negative-working lithographic printing plate is then obtained by executing a development process. In particular, the advantages of excellent productivity, simple development processing, good resolution, and good receptivity are provided by a lithographic printing plate precursor comprising a hydrophilic support provided with a photopolymerizable photosensitive layer containing a photopolymerizable initiator with an excellent photosensitive speed, an addition-polymerizable ethylenically unsaturated compound, and a binder polymer soluble in alkaline development solution, and optionally provided with an oxygen-blocking protective layer, thus providing a plate that exhibits desirable printing characteristics.
The lithographic printing plate precursor described in Japanese Patent No. 2,938,397 is an example of an on-press-developable lithographic printing plate precursor. In this lithographic printing plate precursor, an image-forming layer comprising particles of a hydrophobic thermoplastic polymer dispersed in a hydrophilic binder is disposed on a hydrophilic support. The essential narrative laid out in Japanese Patent No. 2,938,397 is as follows: this lithographic printing plate precursor is photoexposed by an infrared laser in order to bring about image formation by the heat-induced coalescence of the hydrophobic thermoplastic polymer particles and is thereafter installed on the cylinder in the press and can be on-press developed by the fountain solution and/or the ink.
This method of causing image formation by coalescence by the simple thermal melting/bonding of finely divided particles does exhibit an excellent on-press developability; however, the image strength (adhesion to the support) is very weak and the printing durability is thus inadequate.
This on-press developability can be evaluated, for example, in terms of the number of waste sheets of paper, that is, the number of sheets of printing paper required when on-press development is started to reach a state in which ink is not transferred to the nonimage areas.
The lithographic printing plate precursor described in Japanese Patent Application Publication Nos. 2001-277740 and 2001-277742 comprises a hydrophilic support on which there is disposed an image recording layer (heat-sensitive layer) comprising microcapsules that enclose a polymerizable compound.
A lithographic printing plate precursor is described in Japanese Patent Application Publication No. 2002-287334 in which an image recording layer (photosensitive layer) comprising an infrared absorber, a radical polymerization initiator, and a polymerizable compound is disposed on a support.
Due to the high chemical bond density in the image areas, the use of these polymerization reactions does characteristically provide a relatively better image strength than is provided by the image areas formed by the hot melting/bonding of finely divided polymer particles; however, when viewed in terms of practicality, the on-press developability, fine line reproducibility, and printing durability are all still unsatisfactory. In particular, the printing durability using UV inks is very unsatisfactory.
An on-press-developable lithographic printing plate precursor is also described in US Published Application 2003-0064318. This on-press-developable lithographic printing plate precursor comprises a support bearing an image recording layer that contains a polymerizable compound and a graft polymer having pendant polyethylene oxide chains or a block polymer that has a polyethylene oxide block.
The use of this art does provide an excellent on-press developability, but the fine line reproducibility is still inadequate. This fine line reproducibility refers to the reproducibility of a nonimage area interposed between fine lines. In specific terms, using an image chart in which a fine line image of constant width is disposed in alternation with an interposed nonimage area of the same width, the fine line reproducibility refers to the degree to which the nonimage areas between the fine line images on the print are not interrupted by scumming. This characteristic is referred to hereafter as the nonimage area fine line reproducibility or the fine line reproducibility.
In particular, the nonimage area fine line reproducibility is even worse with a UV ink than for a standardly used printing ink (e.g., process ink).
Separately from the preceding, another problem is that components of a photopolymerizable layer that has undergone a typical on-press development form a scum on the dampening roller and ink roller, which impairs the maintenance characteristics of the press and lowers the print quality.
As a method for improving the above conditions, it has been proposed that a sulfonamide group and a hydrophilic group are introduced into a polymer compound, however a printing durability thereby is not sufficient.